The dew point hygrometer is an instrument that measures the air humidity by measuring the dew point temperature. 

Ordinary dew point hygrometer is generally composed of optical mirror, amplifier and power supply. When the air continuously passes through the optical mirror surface, the temperature of the mirror surface is reduced by artificial cooling, so the water vapor in the air condenses on the mirror surface. When condensation occurs, the intensity of the light reflected by the mirror decreases sharply, and the corresponding humidity can be calculated by measuring the temperature (dew point) of the condensation surface at this moment. This type of dew point hygrometer is often used when the temperature is above 0°C. The operating temperature of the lithium chloride dew point hygrometer that has appeared in recent years has expanded to -45°C.